Of all the social networks, one predominates. Facebook is by far the biggest and most influential. When someone or something predominates, it's the largest or most powerful of all.
You'll probably already have noticed that the verb "dominate" makes up a good part of predominate, and in fact these are very similar words. To predominate means to be the biggest or most plentiful or most powerful of all. Of course, you want to make sure that you're talking about like things when you're using predominate. In the debate about guns, the most logical voices are rarely the ones that predominate.